The present invention relates in general to extrusion mixers for the extrusion of molten or fluid plastics, and more particularly to add on or retrofit transfer mixer assemblies of the cavity-transfer type for use in extrusion of polymers in conjunction with the extruder screw of a polymer extruder.
An example of a known cavity transfer type extruder mixer is that disclosed in British Patent Specification No. 930,339 and includes a hollow cylindrical stator member and a cylindrical rotor member which is rotatable within the stator. The facing cylindrical surfaces on the rotor and stator carry respective pluralities of rows of elongate, longitudinally extending grooves. The rows of grooves on each member extend peripherally around the member and are spaced apart axially, the rows on one member being axially offset from the rows on the other member whereby there is axial overlap of the grooves in adjacent rows on the stator and rotor. By virtue of this arrangement of overlapping closed cavities on the rotor and stator, material progressing through this mixer has to travel in a path which alternates between rotor and stator cavities. Where a cavity on one member happens to be opposite a land on the other member the material is subjected to simple shear so that it is cut in half before being displaced approximately at right angles to its original direction as it passes into the next cavity. Another known machine of this type is described in British Patent Specification No. 1,475,216.
It has been found that, whereas such mixers operate satisfactorily when constructed on a small, laboratory scale, when scaled up to a large production size machine the efficiency of the mixer falls, a size limitation being reached where either the product is unsuitable or the added length necessary makes engineering complexity uneconomic. This is because the output rate per screw revolution of an extruder increases in proportion to the cubed power of the screw diameter whereas the available mixer area increases by only the square of the screw diameter if the length to diameter ratio of the mixer is kept constant.
If one attempts to increase the mixing capacity by making the cavities deeper then there is a risk of polymer stagnation and degradation occurring in the corners of the cavities.
Another known type of cavity-transfer type extruder mixer is that disclosed in U.S. Pat. No. 4,419,014 to George M. Gale, which relates to an extruder mixer having facing cylindrical surfaces of a rotor and stator which carry respective parallel rows of hemispherical cavities, wherein the rows of recesses or cavities for each member of the rotor and stator combination are spaced apart axially so that the rows of one member are axially offset from the rows of the other member and there is axial overlap of the recesses in adjacent rows on the stator and rotor.
An object of the present invention is the provision of a fluid mixer, primarily for use in extrusion of polymers in conjunction with a polymer extrusion screw in an extruder, wherein recesses are provided which are continuous grooves, cut on a helix in both the rotor outer diameter and the stator inner diameter, which create a pumping action in both the stator and rotor grooves, and the transfer of fluid from the rotor grooves to the stator grooves and back to the rotor grooves over a specific axial length occurs several times, producing the desired mixing action since the flow from one to the other is continuously being cut off by the rotation of the rotor.
Another object of the present invention is the provision of a fluid mixer as described in the immediately preceding paragraph wherein the grooves cut in the rotor outer diameter and the stator inner diameter change in depth with length on both the rotor and the stator but never disappear, and wherein the elements are not registered circumferentially but are arranged end to end in a random fashion so that a groove in one does not necessarily line up with a groove in the next one, thus allowing random mixing.
Yet another object of the present invention is the provision of a fluid mixer, intended primarily as a retrofit or add-on cavity-transfer type mixer assembly for use with extruder screws and like devices, to serve as a mixing head, wherein the helix angle of the grooves is less than 45.degree. providing a groove pumping capacity which approximates that of an extruder screw metering section.
Other objects, advantages and capabilities of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings illustrating a preferred embodiment of the invention.